The transport of mannitol in Streptococcus mutans by a phosphoenolpyruvate-dependent phosphotransferase system has been demonstrated, and the properties of this enzyme system are being studied in relation to similar transport systems reported for other bacteria. Mutants of Streptococcus mutans unable to grow on mannitol are being studied to elucidate the enzymatic reactions involved in hexitol transport and metabolism. The effect of various metabolic intermediates on the regulation of the phosphotransferase system and on mannitol-1- phosphate dehydrogenase is currently under investigation.